Referring to FIG. 1A, in a wireless communications system, a transmitter 110 and a receiver 120 communicate via a wireless communications channel 150. The transmitter may include an encoder 112 which encodes input information bits into coded bits, and a modulator 118 which modulates the coded bits into a suitable signal format at radio-frequency (RF) frequencies for wireless transmission by the transmitting antenna through the communications channel 150.
In practice, the effects of the communication channel 150 are to distort the RF signal by, for example, introducing multipath effects, noise, timing jitters and frequency offsets. The role of the encoder 112 is to add redundancy to the transmitted data so that errors due to such signal distortions can be corrected after the distorted RF signal is received and demodulated at the receiver 120.
At the receiver 120, a receiving antenna receives the distorted RF signal. The receiver 120 includes a demodulator 122 to demodulate the received RF signal to generate received bits 124. The received bits 124 generally differ from the coded bits 116 due to the signal distortions. The receiver 120 also includes a decoder 126 to decode the received bits. The decoding process generates an estimate 128 of the information bits 114 by essentially reversing the operation of decoder and, in doing so, attempting to recover the information bits 114 in the presence of the signal distortions. The receiver 120 further includes a channel estimator 130 to counteract the effects of the channel 150. FIG. 1A illustrates that the decoder 126 in the illustrated example includes a Viterbi decoder preceded by a block for preparing inputs for the computation of branch metrics. The decoder 126 estimates the information bearing bits, which are re-encoded to allow use as training symbols in channel estimation. The accuracy of the estimated channel will determine the performance of the receiver 120. The transmitter/receiver may include other components which are omitted from FIG. 1A, such as an interleaver/de-interleaver, a scrambler/de-scrambler, and a puncturer/depuncturer.
In the case of an orthogonal frequency division multiplxing (OFDM) based system, the received jth subcarrier in the ith data bearing OFDM symbol of a packet is modelled asr[i,j]=h[i,j]d[i,j]+n[i,j]  (1)where h[i,j] is a complex number representing the frequency domain channel affecting subcarrier j in symbol i, d[i,j] is the symbol sent at the transmitter, and n[i,j] is additive white Gaussian noise (AWGN) affecting the subcarrier. That is, r represents an observation of a received symbol d which has been subject to rotation and scaling (via multiplication by complex-valued h) and additive noise n.
In one implementation, the demodulation includes channel equalisation where the received subcarrier r[i,j] has the channel effects counteracted (or zero-forced) via division as follows:yequalised[i,j]=r[i,j]/h[i,j]  (2).
The equalised or zero-forced observation yequalised[i,j] is then provided to the decoder for decoding.
Reference to any prior art in the specification is not, and should not be taken as, an acknowledgment or any form of suggestion that this prior art forms part of the common general knowledge in any jurisdiction or that this prior art could reasonably be expected to be understood, regarded as relevant and/or combined with other pieces of prior art by a person skilled in the art.